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#Magic maps for mac mac#
The “X” in the binary row represents the bits that are overwritten and shows how 32 multicast IP addresses map to a single multicast MAC address. This means that a host subscribing to a multicast stream could potentially receive multiple multicast streams that it did not subscribe to, and the host will have to discard the unwanted information. This causes a 32-to-1 IP multicast address-to-multicast MAC address ambiguity (2 5 = 32). There’s a small wrinkle in this process, however.ĭid you notice a slight challenge with the number of IP addresses and MAC addresses? Five bits of the IP address are overwritten by the OUI MAC address. End hosts can listen for frames with a specific multicast MAC, allowing them to process only those multicast streams to which they have subscribed. Of the 48 bits used to represent the multicast MAC address, the high-order 25 bits are reserved as part of the OUI, and the last 23 bits of the multicast IP address are used as the low-order bits, as shown in Figure 2-4.įigure 2-4 Layer 2 Multicast Address FormatĪ switch can use this calculated multicast MAC address to distinguish a frame as a multicast and make efficient forwarding decisions. The high-order 4 bits for the destination IP multicast address are set to 1110 binary (0b1110). The lower-order 23 bits of the destination IP multicast address are mapped to the lower-order 23 bits of the MAC address. These bits are part of the organizational unit identifiers (OUI). The high-order 25 bits is the official reserved multicast MAC address range from 0100.5E00.0000 to (request for Comment 1112). The sending device must convert the destination IP multicast address into a special MAC address as follows: What happens when it is a multicast message? To optimize network resources, an Ethernet switch also needs to understand multicast.
As discussed in Chapter 1, when a device sends a broadcast frame, the destination address is all ones, and a unicast message is the destination MAC address. A traditional Ethernet switch (Layer 2 device) works with Ethernet frames, and a traditional router (Layer 3 device) looks at packets to make decisions on how messages will be handled.
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